The present invention relates to a method for reading radiation image from a stimulable phosphor sheet in which a latent radiation image is contained, and an apparatus for the radiation image reading method.
When a stimulable phosphor is exposed to a radiation such as X-ray, it absorbs and stores a portion of energy of the radiation. The stimulable phosphor then emits stimulated emission according to the level of the stored energy when the phosphor is exposed to stimulating light.
A radiation image recording and reproducing method utilizing the stimulable phosphor has been widely employed in practice. This method utilizes a stimulable phosphor sheet (also called, radiation image storage panel), and comprises the steps of causing the stimulable phosphor of the phosphor sheet to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with a stimulating light such as a laser beam to emit a stimulated emission; and photoelectrically collecting the stimulated emission to convert it into electric signals. The stimulable phosphor sheet thus processed is then subjected to a step for erasing radiation energy remaining therein, and then stored for performing the next image recording and reproducing procedure. Thus, the stimulable phosphor sheet can be repeatedly used.
The stimulable phosphor sheet has a basic structure comprising a support and a stimulable phosphor layer provided thereon. On the free surface (surface not facing the support) of the phosphor layer, a protective film is generally placed to keep the phosphor layer from chemical deterioration or physical damage.
The stimulable phosphor layer generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The phosphor layer containing no binder can be formed by firing process. Further, the stimulable phosphor layer can be produced by a gas phase deposition method.
A modified process for reading the radiation image in the form of simulated emission is also proposed so as to shorten the time for reading, to downsize the apparatus, and to reduce the production cost. In the-proposed process, the radiation image is divided to form pixels on a photodetector (such as a two-dimensional solid image pickup device or a semiconductor line sensor) to obtain a time-dependent series of image signals through an electric circuit. Also known is an apparatus for performing a process comprising the steps of exposing a stimulable phosphor sheet to a stimulating light emitting from a light source (e.g., a fluorescent lamp) through a slit for linearly stimulating the phosphor sheet (that is, line stimulation), and collecting the stimulated emission emitting from the phosphor sheet by a line sensor which is composed of a number of photoelectric converting elements (that is, line detection). The line sensor is employed under the condition that it would face the stimulating light-exposed area of the phosphor sheet.
In the conventional procedure for reading the stimulated emission, a light-collecting guide and a photodetector such as a photomultiplier are used in combination. Since the emission coming out of the area exposed to the stimulating light (such as laser beam) is almost completely collected by the light-collecting guide, the image quality such as sharpness depends on the amount of the emission coming from the phosphor sheet. This means that the image quality depends on the thickness of the stimulable phosphor layer. It is, therefore, important to prepare the phosphor layer having an even thickness in order to obtain a radiation image having even image quality.
In the aforementioned modified process, a light-collecting lens such as a SELFOC lens array is used for efficiently collecting the stimulated emission onto the line sensor. The stimulated emission is collected and condensed by the light-collecting lens so that an image would be formed on the light-receiving plane of the line sensor.
It has been now found that for obtaining a clear image, the distance between from which the stimulated emission emits and the emission-receiving plane should be in a specific range. Particularly, if the distance between the light-collecting lens and the surface of the phosphor sheet varies even slightly, the area sighted by the lens so varies that the light-collecting efficiency considerably varies. Consequently, even the slight variation of the distance between the emission-receiving plane and the surface of the stimulable phosphor layer impairs the image quality such as sharpness considerably. The inventors have thus found that in the line detection, the image quality depends on the variation of the distance between the emission-receiving plane of the radiation image-reading means and the surface of the stimulable phosphor layer rather than the thickness of the stimulable phosphor layer.
The inventors have further studied the quality of radiation image obtained by the line detection, and found that the surface irregularities of the stimulable phosphor sheet, particularly the surface irregularities of the stimulable phosphor layer impairs the image quality. The stimulable phosphor layer is formed by, for example, a coating method or a gas phase deposition method, but by any method it is almost impossible to make the surface having no irregularities. In fact, on the surface of the phosphor layer, there usually are produced both microscopic unevenness (roughness in the order of 1 to 100 xcexcm) and macroscopic irregularities (e.g., irregularities of the thickness of the phosphor layer). The microscopic unevenness may not affect the image quality because it is within single pixel corresponding to each photoelectric converting element of the line sensor. The macroscopic irregularities, however, cause variation of the distance between the emission-receiving plane and the surface of the stimulable phosphor layer, and accordingly sometimes give a radiation image having uneven image quality.
In the conventional radiation image-reading apparatus, a stage on which the stimulable phosphor sheet is placed is fixed and the distance between the stage and a stimulated emission-collecting means (comprising the emission-collecting lens and the line sensor) is fixed. However, it has been found that the distance between the stimulable phosphor layer and the stimulated emission-receiving plane cannot be satisfactory kept at a constant length only by fixing the phosphor sheet onto the stage because the thickness of the stimulable phosphor layer is not strictly the same in the whole area.
An object of the present invention is to provide an improved method for reading radiation image from a stimulable phosphor sheet in which a latent radiation image is contained.
Another object of the invention is to provide an apparatus for performing the radiation image reading method of the invention.
The present invention resides in a method for reading a radiation image from a stimulable phosphor sheet comprising a transparent substrate and a stimulable phosphor layer containing therein a latent radiation image by means of a radiation image-reading means comprising a stimulating light-applying unit and a stimulated emission-collecting unit comprising a lens and a stimulated emission-receiving plane, which comprises the steps of applying a stimulating light onto the stimulable phosphor layer under the condition that the stimulable phosphor sheet moves along a sheet plane thereof in relation to the stimulated emission-collecting unit; collecting a stimulated emission emitting from the area onto which the stimulating light is applied on the stimulated emission-receiving plane through the transparent substrate and lens; and photoelectrically converting the collected emission into electric signals in the stimulated emission-collecting unit, wherein the stimulable phosphor sheet moves in relation to the stimulated emission-collecting unit under the condition that the stimulating light-applied area of the stimulable phosphor layer is kept apart from the center of the stimulated emission-receiving plane with a space in the range defined by a combination of a reference space and a focal depth of the lens, the reference space being defined by a length at which the stimulated emission emitting from the stimulable phosphor layer focuses on the stimulated emission-receiving plane after passing through the transparent substrate and lens.
In the above-mentioned method of the invention, it is preferred that the stimulable phosphor sheet has a reference plane area on both side surfaces of the transparent substrate on the side of the stimulable phosphor layer and the stimulable phosphor sheet is moved under the condition that the phosphor sheet is supported at the reference plane areas on a supporting means which is arranged in a position fixed in relation to the stimulated emission-collecting unit.
Also preferred is that the stimulable phosphor sheet has a reference plane area on both side surfaces of the transparent substrate on the side of the stimulable phosphor layer, the stimulable phosphor sheet is supported at the reference plane areas on a supporting means, and the stimulated emission-collecting unit is moved on the supporting means.
In the above-mentioned method of the invention, it is preferred that the stimulating light-applying unit and stimulated emission-collecting unit of the radiation image-reading means are arranged on the side facing the transparent substrate of the stimulable phosphor sheet.
The present invention also resides in a radiation image reading apparatus for performing the radiation image reading method of the invention, which comprises the radiation image reading means, the supporting means which is arranged in a position fixed in relation to the radiation image-reading means and allows movement of the stimulable phosphor sheet along the sheet plane by supporting the stimulable phosphor sheet at the reference plane areas, and a driving means for driving the movement of the stimulable phosphor sheet.
The present invention also resides in a radiation image reading apparatus for performing the radiation image reading method of the invention, which comprises the radiation image-reading means, the supporting means which is arranged in a position fixed in relation to the stimulable phosphor sheet and allows movement of the radiation image-reading means, and a driving means for driving the movement of the radiation image-reading means.
The present invention further resides in a method for reading a radiation image from a stimulable phosphor sheet comprising a substrate and a stimulable phosphor layer containing therein a latent radiation image by means of a radiation image-reading means comprising a stimulating light-applying unit and a stimulated emission-collecting unit comprising a lens and a stimulated emission-receiving plane, which comprises the steps of applying a stimulating light onto the stimulable phosphor layer under the condition that the stimulable phosphor sheet moves along a sheet plane thereof in relation to the stimulated emission-collecting unit; collecting a stimulated emission emitting from the area onto which the stimulating light is applied on the stimulated emission-receiving plane through not the substrate but the lens; and photoelectrically converting the collected emission into electric signals in the stimulated emission-collecting unit, wherein the stimulable phosphor sheet moves in relation to the stimulated emission-collecting unit under the condition that the stimulating light-applied area of the stimulable phosphor layer is kept apart from the center of the stimulated emission-receiving plane with a space in the range defined by a combination of a reference space and a focal depth of the lens, the reference space being defined by a length at which the stimulated emission emitting from the stimulable phosphor layer focuses on the stimulated emission-receiving plane after passing through the lens.
In the above-mentioned method of the invention, it is preferred that the stimulable phosphor sheet has a reference plane area on both side surfaces of the substrate on the side of the stimulable phosphor layer and the stimulable phosphor sheet is moved under the condition that the phosphor sheet is supported at the reference plane areas on a supporting means which is arranged in a position fixed in relation to the stimulated emission-collecting unit.
Also preferred is that the stimulable phosphor sheet has a reference plane area on both side surfaces of the substrate on the side of the stimulable phosphor layer, the stimulable phosphor sheet is supported on a supporting means, and the stimulated emission-collecting unit is moved on the reference plane areas of the stimulable phosphor sheet.
The invention also resides in a radiation image reading apparatus for performing the above-mentioned radiation image reading method of the invention which comprises the radiation image-reading means, the supporting means which is arranged in a position fixed in relation to the radiation image-reading means and allows movement of the stimulable phosphor sheet along the sheet plane by supporting the stimulable phosphor sheet at the reference plane areas, and a driving means for driving the movement of the stimulable phosphor sheet.
The invention also resides in a radiation image reading apparatus for performing the above-mentioned radiation image reading method of the invention which comprises the radiation image-reading means, the supporting means which is arranged in a position fixed in relation to the stimulable phosphor sheet, and a driving means for driving the movement of the radiation image-reading means on the reference plane areas of the stimulable phosphor sheet.
The invention furthermore resides in a stimulable phosphor sheet comprising a rigid substrate, a stimulable phosphor layer having irregularities on a surface thereof, and a reference plane-forming guide means attached to the rigid substrate on both side surfaces thereof, the reference plane-forming means having a surface of irregularity identical to those of the surface of the stimulable phosphor layer.
In the stimulable phosphor sheet, if the stimulable phosphor layer has a curved, deformed or sloping surface thereon, the reference plane-forming guide means should have a surface identical to the surface of the stimulable phosphor layer
The stimulable phosphor sheet of the invention preferably is in the form of one of the following embodiments:
1) The transparent substrate has a reference plane area on which the surface has irregularities within xc2x1100 xcexcm in terms of a mean irregularity height.
2) The transparent substrate is a rigid transparent substrate such as glass sheet.
3) The stimulable phosphor layer has a stimulated emission-reflecting layer on the side not facing the substrate.
4) The stimulable phosphor layer is produced by a gas phase deposition method.
5) The stimulable phosphor sheet has on a surface of the stimulable phosphor layer irregularities within xc2x150 xcexcm in terms of a mean irregularity height.
6) The substrate is a rigid substrate.
7) The rigid substrate is made of material having a modulus of elasticity of 1xc3x97105 kgf/cm2 or higher and a thickness in the range of 200 xcexcm to 10 mm, preferably 100 xcexcm to 10 mm.
8) The substrate is placed on a rigid auxiliary substrate, which is made of material having a modulus of elasticity of 1xc3x97105 kgf/cm2 or higher and a thickness in the range of 200 xcexcm to 10 mm, preferably 100 xcexcm to 10 mm.